In the past, several drugs having a tricyclic structure have been found useful in the treatment of severy psychotic disorders, especially of the schizophrenic type. Most of these derivatives are phenothiazine derivatives which are substituted in the 2-position of one of the benzene rings and having at the ring nitrogen atom an alkyl side chain substituted with a tertiary amino group at a position three carbon atoms from the ring nitrogen atom. The tertiary amino group may also form part of a heterocyclic ring system and such a system, especially the piperazine ring system, is present in several very strong neuroleptic drugs. Also thiaxanthenes substituted in equivalent positions with similar groups and having an unsaturated bond have been found useful in the treatment of psychoses. It is a well-known fact that the aforementioned mono-substituted phenothiazines or thiaxanthenes cause severe extrapyrimidal symptoms in many patients, which makes further treatment difficult or impossible.